Contact of copper with brass and tin layers



April 13, 1967 H. HOFFMANN ETAL 3,314,771

CONTACT OF COPPER WITH BRASS AND TIN LAYERS Filed March 11, 1964 UnitedStates Patent Germany Filed Mar. 11, 1964, Ser. No. 351,037 Claimspriority, application Germany, Sept. 11, 1963, L 45,823, L 45,824

4 Claims. (Cl. 29-183.5)

provided with a coating whose pu pose it is to maintain the contactresistance of the contact element constant.

Contact elements of the type used in switching equipment, i.e., theelements whose surfaces are used to establish the actual galvaniccontact, are conventionally made of a material which has a low specificresistance. The contact surfaces should remain metallically clean for aslong as possible so that, irrespective of the contact pres sure, thecontact resistance, which results in a voltage drop across the contactelement and in a heat loss, is not unduly increased but remains withingiven tolerances. It

alloys, are preferred. Such contact elements, however, have the drawbackthat they are vulnerable to sulphur or sulphur-containing substances.This drawback becomes particularly significant in the case of contactelements associated with switching equipment which is not operated veryfrequently, so that the contacts are not subjected to the automaticwiping and self-cleaning action which is built into many types ofswitches. Contact elements made of a base metal, as, for example,copper, and provided with a coating of a noble metal, such as silver,are subjected to the same drawback.

Other attempts have been made to render contact elements more resistantto tarnishing. One way of doing this is to convert the surface of thecontact elements into a chromate compound. But here, too, sulfide layerswill, in the course of time, be formed, particularly if the switchoperates in an atmosphere of agressive media, which sulfide layersincrease the contact resistance.

Another way of reducing tarnish has been to use, instead of coatings ofa noble metal, a coating made of a base metal, as, for example, tin.Coatings of this type element, and experience has contact element, i.e.,the voltage drop across it, becomes progressively worse.

German Patent No. 830,269 shows slide bearings provided with tin runningsurfaces. In order to prevent the tin from diffusing into the basicmetal, layers of iron,

ning in of the slide bearings.

In the case of tin-plated copper contact elements oper- 3,314,771Patented Apr. 18, 1967 ating in a sulphurous atmosphere, no sulfide isformed and the voltage drop does not increase. Such a contact elementwill, however, operate satisfactorily only so long as it is subjected torelatively low temperatures, i.e., so long as the contact element issubjected to low loads. during operation, the normal operatlngtemperature becomes higherdue, for example, to increased contactresistance resulting from low contact pressure the operating temperaturewith a coating made of tin, this coating being applied onto a barrierlayer which is between the coating and the basic metal, which barrierlayer prevents the tin from diffusing into the basic metal so that theelectrical characteristics of the contact element remain unchanged. Ifthe barrier layer were made of iron, nickel and cobalt and the alloysthereof (see above-mentioned German Patent No. 830,- 269), such barrierlayers these layers would, of FeSn and Ni Sn which would increase theelectrical resistance. Therefore, the dilfusion barrier layers used incontact elements according to the instant invention are as brass,tin-zinc, and tin-lead.

crease even at relatively high operating temperatures of, for example,200 C.

The provision of the barrier layer also prevents the discoloration ofthe tin coating which is a normal conappa-rent upon consideration of thefollowing description when taken in conjunction with the accompanyingdrawings in which:

FIGURE 1 is a sectional view of a contact element according to thepresent invention which is provided with multiple layers and coatings.

FIGURE 2 is a sectional view of a contact: element according to theinstant invention which is provided with mgs.

Referring now to the drawings and to FIGURE 1 thereof in particular, thesame shows a contract element whose basic slab or body portion 10 ismade of a base metal such as copper. The basic portion 10 has applied toit a barrier layer 2 made of brass ii. of 63% copper, 37% zinc, atin-zinc alloy f.i. of tin, 25% zinc, or a tin-lead alloy f.i. of 10%tin, lead. Applied over the barrier layer 11 is the tin layer 12, bothof layers 11 and 12 being applied galvanically.

The contact element as described so far is rendered even moreoxydation-resistant, even when subjected to higher temperatures forextended periods of time, by applying a plurality of sets of barrierlayers and tin coatings, i.e., by applying to the basic metal a barrierlayer, a tin coating, a further barrier layer overlying the first tincoating, a further tin coating overlying the second barrier layer, andso on. Even though a tin coating is not, or not always, absolutelyporous-free, a medium tending to cause deterioration of the contactelement will, despite the most unfavorable conditions, be prevented fromadversely affecting the surface of the contact element inasmuch as thereare a plurality of barrier layers which prevent the tin coating fromdiffusing into the basic metal.

FIGURE 1 thus shows a five-layer contact element, consisting not only ofthe copper base 10, and a first brass barrier layer 11, a first tincoating 12, but also of a second brass layer 13, and a second tincoating 14. The various layers and coatings are applied galvanically,one on top of the other so that all of the layers and coatings arebonded together.

The contact element of FIGURE 1 can be used as a pressure-type contactelement, as an inserta-ble slide-type contact element, or as a contactblade. As stated above, the advantages realized by the present inventionare of particular significance for use in switch gear which is operatedbut infrequently.

FIGURE 2 shows a multiple layer contact element in which at least one ofthe layers serves as a diffusion layer which does not diffuse with thebasic metal (copper, copper alloy, or iron), but with the material ofwhich the coating is made, namely, the tin. As a result, even the mostunfavorable conditions (aggressive sulphurous atmosphere, hightemperature and long exposure thereto, slightly porous tin coating) willnot result in any free copper appearing at the contact surface of thecontact element.

In the multiple-layer arrangement intermetallic phrases rather than puremetals or alloys are used as the barrier layers. Here, the first barrierlayer 21 applied to the basic metal 20, e.g., copper or brass, is ametal which can not form any diffusion layer but a mixed crystal, as,for example, silver. Applied over layer 21 is a layer 22 of tin whichcan form a silver diffusion layer with the layer 21. The third layer 23is brass, and the fourth layer 24 is the final tin coating. The contactelement of FIGURE 2, comprising the basic copper or brass, thus has amultiple layer arrangement of silver-tin-brass-tin, the thickness of thesilver being between 1 and microns and preferably 5 microns, thethickness of the first tin layer being between 5 and 15 microns andpreferably microns, the thickness of the brass layer being between 1 and5 microns and preferably 5 microns, and the thickness of the final tinlayer being between 5 and 20 microns and preferably microns.

Before use, the contact element is heat-treated at 200 C. for a periodof between 6 and 200 hours and preferably 48 hours, thereby to makecertain that the difiusion layer is formed between the silver and thetin. A thus-formed contact element has been found to be suitable for usein sulphur-containing atmospheres because the intermetallic phases ofsilver and tin are stable.

The present invention is not strictly limited for use with contactelements but is applicable for other types of current conductors aswell; moreover, the present invention is generally applicable whereverthe corrosion of a tin coating is to be prevented.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

What is claimed is:

1. A contact element comprising a body made of copper, a first brasslayer on and bonded to said body, a first tin coating on and bonded tosaid first brass layer, a second brass layer on and bonded to said firsttin coating, and a second tin coating on and bonded to said second brasslayer.

2. A contact element comprising a body made of copper, a silver layer onand bonded to said body, a first tin layer on and bonded to said silverlayer, a brass layer on and bonded to said first tin layer, and a secondtin layer on and bonded to said brass layer.

3. A contact element as defined in claim 2 wherein said silver layer hasa thickness of between 1 and 5 microns, wherein said first tin layer hasa thickness of between 5 and 15 microns, wherein said brass layer has athickness of between 1 and 5 microns, and wherein said second tin layerhas a thickness of between 5 and 20 microns.

4. A contact element as defined in claim 2 wherein said silver layer hasa thickness of 5 microns, wherein said first tin layer has a thicknessof 10 microns, wherein said brass layer has a thickness of 5 microns,and wherein said second tin layer has a thickness of 15 microns.

References Cited by the Examiner UNITED STATES PATENTS HYLAND BIZOT,Primary Examiner.

1. A CONTACT ELEMENT COMPRISING A BODY MADE OF COPPER, A FIRST BRASSLAYER ON AND BONDED TO SAID BODY, A FIRST TIN COATING ON AND BONDED TOSAID FIRST BRASS LAYER, A SECOND BRASS LAYER ON AND BONDED TO SAID FIRSTTIN COATING, AND A SECOND TIN COATING ON AND BONDED TO SAID SECOND BRASSLAYER.